The pressure within the eyeball is naturally maintained by a continuous flow of aqueous humour produced by ciliary body. It drains out the excess eyeball fluid through channels called the trabecular meshwork. If the outflow is blocked, the aqueous humour builds up inside the eye, increasing the pressure within the eyeball. This pressure needs to be reduced, as otherwise it can damage the optic nerve, resulting in an optic neuropathy and irreversibly impair vision as a result. This condition is known as glaucoma—a disease that affects more than 60 million worldwide and is the second leading cause of blindness. Intraocular pressure (IOP) remains the key modifiable risk factor in glaucoma.
Latanoprost is a prostaglandin analogue and is a potent drug that can reduce the IOP of the eye by increasing aqueous outflow through the uveoscleral pathway. The ester bond of latanoprost gets easily hydrolyzed forming IPA (isopropyl alcohol) and latanoprost acid, which is more hydrophilic and experiences a higher penetration resistance through the epithelium and endothelium of the cornea. Thus, this causes stability problems for this drug in aqueous, low and high pH range. To avoid this instability, coupled with the low solubility, it is mainly delivered in the form of, for example, oil/water emulsion, and lipid/buffer emulsion (for example, Xalatan). There is another unavoidable issue of irreversible yellow pigmentation of corneal epithelium after regular application of commercial eye drop for longer period (for example, beyond three months). This pigmentation is attributed to the presence of benzalkonium chloride, which acts as a preservative for latanoprost in the commercial eye drop.
In addition to the stability problem of the drug, there exist several challenges in ocular drug delivery, e.g. (i) heavy tear drainage cum dilution mechanism washes away any substance from corneal/conjunctival epithelium cell; (ii) the trilaminate corneal epithelium has a stromal cell layer sandwiched between highly lipophilic outer layer and hydrophilic inner layer and each of these layers contributes to resistance of various amount depending on the lipophilicity of drug. Further, of the two common pathways (paracellular and transcellular) for the transport of drug molecules, the drug molecules would predominantly use the transcellular path to cross the cornea, where the lipophilicity and dissociation constant (pKa) are the major parameters that decide the entry of drug molecules.
Because of all these factors, a mere 5% of the free drug applied on the corneal epithelium successfully penetrates through the cornea. Thus, the effective drug concentration reaching the aqueous humor often falls below the therapeutic limit and repeated administration is necessary because a substantial portion goes into the conjunctival sac and enters the circulation, causing undesirable side effects. In order to improve bioavailability of drug, the transport barrier as well as prolonging the retention of the drug carrier in the anterior segment of the eye would be necessary.
Specifically, for the drug latanoprost, which is currently administered by daily eye drops, patient compliance and adherence to the strict regimen is a serious matter. Daily application, poor ocular bioavailability of topical application of drugs, and other long-term side effects affect patient compliance, which leads to disease progression.
Sustained topical delivery in the front of the eye is made difficult because of drainage. Therefore, formulations like eye drops, ointments although easy to apply get cleared away with only a proportion of the drug getting transported through these barriers. Therefore, to avoid (i) frequent administration and (ii) undesirable toxic effects of drug, liposomal formations might be a possible and more favoured alternative delivery system. The liposome is a lipid vesicle that possesses a hydrophilic core and hydrophobic boundary wall along with potential for specificity by surface modification. This helps liposomes to become effective for delivering a wide range of drugs and prevents the drug from being degraded by external physiological conditions. Delivery of ocular drugs using liposomal formulation relates to various problems associated with drug penetration, stability, efficacy, and sustainability. However, delivery of a hydrophobic drug due to its low water solubility limit is considerably more challenging. The limited bilayer space availability in the lipid bilayer often limits the loading of a lipophilic drug and is the most significant hurdle in developing liposomes for sustained delivery of hydrophobic entities, including latanoprost.
A transparent drug loaded liposomal formulation that can evade mononuclear phagocytic uptake may provide as a good alternative. Liposomes, which have been shown to be biocompatible nanocarriers for ocular use, allows for delivery of both the lipophilic drug molecule as well as its hydrophilic active products, due to its physical structure of a polar core and lipophilic bilayer. Liposomal encapsulation protects drug molecules from enzymatic hydrolysis in the physiological environment while in circulation, and thus increases stability.
However, the size of small uni-lamellar vesicle (SUV, 20-50 nm) or large uni-lamellar vesicle (LUV, ˜100 nm) often restricts its transport through epithelial layers, and also permits rapid clearance during topical administration. Thus, several modifications of liposomes have been reported: (i) surface modification with charged lipids which can make vesicles adhere to the oppositely (negatively) charged corneal epithelium, (ii) increasing the lipophilicity of the drug molecules, and (iii) modifying the integrity of corneal epithelium transiently by using a penetration enhancer. Such modifications would facilitate the first stage of the entry of drug molecules, while the subsequent permeation would be governed by pKa, hydrophilicity/lipophilicity of the drug molecules.
Various routes of administration of liposomal formulations comprise topical administration, intravitreal, subconjunctival and systemic injection.
Previous studies on topical application of liposomes demonstrated poor penetration into the eye; while studies on subconjunctival injections on other IOP-lowering drugs show limited sustainability.
For example, in in vivo studies for topically applied liposome loaded drug formulations, positively charged egg phosphatidyl choline (PC): cholesterol (CH): stearyl amine (7:4:1) multilamellar vesicles (MLVs) loaded with acetazolamide, when applied topically, shows the highest lowering of IOP (−7.8 mmHg) after 3 hours. However this lowering lasts only for about 8 hours.
Comparing sub-conjunctival delivery of 6-carboxyfluorescein (as a “model” drug) in rabbit eyes in aqueous solution and liposomal formulation, the released drug concentration is noticeably high after 30 mins and transitory conjunctivitis (i.e., common swelling due to excess drug concentration) is observed, but no side effect has been reported. The liposomal formulation shows detectable levels of carboxyfluorescein at the injection site of sub-conjunctiva along with sclera, cornea, choroid and retina after 7 days of injection, but the reason attributing to the sustainability/retention in this study has not clearly been understood. Furthermore, it is also not clearly understood whether drug-loaded liposomes vesicles in the sub-conjunctiva behave like a depot system or actually undergo circulation and endocytosis.
In vitro application of norfloxacin loaded liposome onto cornea has improved drug retention when compared to free drug solution. They indicate a probable corneal endocytic uptake of norfloxacin loaded liposome through the corneal membrane. There appears to be no other report of enhanced drug retention in the anterior segment of the eye. The pharmacokinetics of drug release from the liposome vesicle and penetration through corneal epithelium is still poorly understood. If the drug release from these vehicles is appreciably slow, then clearance will dominate drug adsorption and diffusion through corneal epithelium. Therefore, released drug will be too diluted (tear dilution) to have an adequate gradient to cross the corneal epithelium. Thus, it may perform poorer than even free drug.
Although liposomal carriers have been evaluated for delivery of drugs topically on the front of the eye, none have been successful to date for sustained release. Very few studies have been reported on the fate of liposomes injected subconjunctivally. Nevertheless, subconjunctival injection may be an attractive option for sustained delivery of anti-glaucoma agents, provided the effects can be sustained for at least 1 month, or preferably longer. Specifically, for the delivery of latanoprost (a prostaglandin derivative that is currently administered via once-a-day eye drops), there is no report on comparison of in vitro and in vivo release behavior.
Thus it is an object of the present invention to provide a liposomal formulation for ocular drug delivery that improves the sustained release of the drug and circumvents the problems presented by the strict regime of patient compliance and adherence.